Zeolites or molecular sieves are three-dimensional crystalline alumina-silicates with internal pore structure having both cation exchange capacity and adsorption capacity. The cation exchange capacity of zeolites is related to the aluminum content and pore size, the aluminum atom carrying a unit negative charge in the crystalline framework and thus determining the cation exchange capacity. If the hydrated radius of a cation is too large for diffusion through the pores, then exchange capacity and rate for that cation will be low. The adsorption capacity of zeolites is related to the pore size and the size of the adsorbate molecule. Materials which can be diffused into the pores will be adsorbed while larger ones will not.
Zeolites may be synthesized from concentrated silica-alumina gels under basic conditions such as described in U.S. Pat. No. 2,882,243. According to U.S. Pat. No. 3,663,165, a zeolite molecular sieve catalyst has been synthesized from kaolin clay by calcining the kaolin at 1800.degree. F. for 2 hours to form m-kaolin followed by two treatments with aqueous sodium hydroxide. The first treatment is carried out at 100.degree. F. for 12 hours while the second is carried out at 180.degree. F. for 12 hours.
U.S. Pat. No. 4,271,130 also describes a process for the preparation of Zeolite A from kaolin by converting the kaolin to meta-kaolin by flash heating at specific residence times and annealing in the presence of alkaline earth compounds, an uncolored halide or halogen and an alkali metal compound and reacting the meta-kaolin in an aqueous alkaline medium.
Certain natural zeolites do occur as mineral deposits in certain parts of the world. These mineral zeolites are usually found with varying amounts of other impurities and generally have low aluminum content. A natural mineral or natural zeolite is heulandite having the formula Ca1/2.sub.x (AlO.sub.2).sub.x (SiO.sub.2).sub.y.ZH.sub.2 O wherein x=5 to 9, x/y=2.5 to 4.0 and Z=1 `to 30.
A process for the preparation of zeolite A by hydrothermal treatment of clinoptilolite having the formula Na.sub.6 (AlO.sub.2).sub.6 (SiO.sub.2).sub.30.24H.sub.2 O in a slurry of sodium aluminate and aqueous sodium hydroxide is described in J. J. Leonard, U.S. Pat. No. 4,247,524. Due to the fact that natural clinoptilolite contains varying amounts of elemental impurities which are either insoluble under such reaction conditions or are exchangable with the sodium form of the zeolite A product an undesirable off-white product having a low cation exchange capacity will usually result. In addition, control of the zeolite A particle size is difficult.
Heulandite is one of the natural mineral zeolites of a group of hydrous alkali and/or alkaline earth alumino-silicates which have an open three-dimensional crystalline framework. The chemical and physical properties of heulandite, as well as of zeolite A, is described in Breck, Zeolite Molecular Sieves (1974) and other publications. The geologic occurence of natural mineral zeolites and some industrial and agricultural uses of zeolites in general are given. Zeolites including the zeolite A prepared by the instant process may be employed in various catalyst and adsorption systems.
It is a principle object of this invention to provide an improved process which produces a sodium cation form of zeolite A from heulandite in high yield and of superior quality including color and particle size distribution.